Liquid ring pumps are well known. U.S. Pat. No. 4,850,808, Schultze, discloses such a liquid ring pump. The pump has one or two stages. The pump includes an annular housing; a rotor assembly within the annular housing; a shaft extending into the annular housing on which the rotor assembly is fixedly mounted; and a motor assembly coupled to the shaft. During operation, the annular housing is partially filled with operating liquid so that when the rotor is rotating, the rotor blades engage the operating liquid and cause it to form a liquid ring that diverges and converges in the radial direction relative to the shaft. Where the liquid is diverging from the shaft, the resulting reduced pressure in the spaces between adjacent rotor blades of the rotor assembly (buckets) constitutes a gas intake zone. Where the liquid is converging towards the shaft, the resulting increased pressure in the spaces between adjacent rotor blades (buckets) constitutes a gas compression zone.
U.S. Pat. No. 4,251,190, Brown discloses a water ring rotary air compressor. The compressor includes an annular housing; a rotor assembly disposed within the annular housing; a motively powered shaft extending into the annular housing and fixedly coupled to the rotor assembly. The rotor assembly utilizes a pumping liquid and creates a liquid ring in a manner similar to U.S. Pat. No. 4,850,808.
Through prolonged use of such pumps, the liquid ring may cause corrosion of the surfaces of the annular housing that are in contact with the liquid ring. For example, the annular housing may experience corrosion erosion, cavitation erosion, and/or particle erosion. Over time the corrosion roughens the wetted surfaces of the annular housing thereby increasing a frictional drag of the liquid ring along the surface of the annular housing. The increased drag requires an increase in the amount of power that is necessary for the shaft to properly operate the pump. Accordingly, the efficiency and life-span of the pump is decreased. For example, tests conducted on a 7.5 Hp vacuum pump operating at 1750 rpm show that over 10-15 weeks of operation the annular housing surface roughness increased so much that to maintain the 1750 rpm operating speed, shaft power had to be increased by as much as 6.2%. Some known liquid ring pumps have addressed the issue of corrosion and annular housing surface roughness by forming annular housings from corrosion resistant casting materials, such as cast stainless steel. However, the cost of cast stainless steel is several times the cost of cast iron thereby making this approach uneconomical.